It is well-known that two counter-rotating rolls thrust against each with a determined force S and subjected to reciprocal crossing by a determined angle α (FIG. 1), even limited to a few tenths of a degree, with respect to the vertical plane passing through the median rolling axis, are also subject to axial thrusts.
In the zone of contact between the working rolls in a rolling stand, because of the sliding speeds VS=V′−V (vectorial difference of the peripheral speeds of the rolls), where V is the peripheral speed of the upper roll and V′ is the peripheral speed of the lower roll, and because of the contact friction due to the force S, axial forces T are produced on the upper roll and T′ on the lower roll, which can even be of strong intensity, if the force S is high.
Therefore, a rolling stand equipped with a system to cross the rolls such as to generate an angle α between two rolls in contact generates a thrust T, which changes sign when the rotation of the rolls is reversed, and which is discharged onto the respective thrust bearings A and B.
In the case of a six-high rolling stand, the two intermediate rolls (IR), pressed with the force of separation S by the corresponding back-up rolls (BUR) and working rolls (WR) on opposite generatrixes, if they are inclined, that is crossed, by the same angle anti-symmetrically, induce axial forces of the opposite sign on the back-up rolls and the working rolls.
The thrusts which can occur are transmitted by means of the thrust bearings to the respective chocks, which discharge them on the bilateral axial holding elements (called chock gates) which discharge the forces onto the housings of the stand.
It should be noted that the intermediate rolls are, in theory, exempt from axial thrust, since each is loaded, on the two opposite generatrixes, with equal axial forces of the opposite sign, if the friction coefficients on the two contacts are equal.
Consequently, it is necessary to adopt low-friction holding elements (chock gates), both for the working rolls and for the back-up rolls, in order to contain the vertical friction forces which generate hysteresis which impedes the small vertical movements of the roll pack during rolling.
The axial forces are discharged onto the chock, which transmits the load to the chock gates which clamp the chock axially.
To regulate the thickness of the rolled product, it must be easy to position the rolling rolls vertically.
The reaction of the chock gates, multiplied by the coefficient of friction between the chock and the chock gate, generates a vertical force which opposes the movement of vertical positioning of the rolls. Thus a hysteresis is created which has a harmful effect on the correct control of the thickness of the rolled product.